When a yaw drive is operated, a motor provides high speed rotation for an output shaft. The output shaft is connected to an input shaft of a gear assembly. In the gear assembly the rotational speed is reduced, and an output gear wheel of the gear assembly is arranged in engagement with a large gear ring, thereby causing a nacelle of a wind turbine to rotate about a substantially vertical axis.
The motor of the yaw drive is often provided with a brake, such as an electro-magnetic brake, which is used when a yawing movement is to be stopped, and when a yawing position is to be maintained. However, such brakes tend to be inaccurate, thereby providing a too high braking force and introducing loads in the yaw drive, and possibly in the entire yawing system, and even in the entire wind turbine. In order to reduce such loads, some yaw drives have been provided with torque limiters.
A torque limiter allows torque to be transferred between two rotatable parts under normal operating conditions. But in the case that an applied torque exceeds a predefined threshold value, the torque limiter ensures that only torque corresponding to the threshold value is transferred, i.e. the torque being transferred is limited. The torque exceeding the threshold value is absorbed by the torque limiter. Thereby it can be avoided that damaging torque loads are transferred through the system.
Previously torque limiters have been arranged in gear systems, e.g. between two gear stages. One disadvantage of this is that it is necessary to disassemble the gear system in order to provide service or adjustment to the torque limiter.
EP 1 793 143 A1 discloses a gear reducer comprising a torque limiter. The torque limiter is designed to transmit torque, or, once a given threshold value of the torque is exceeded, to cause a crown wheel to slip with respect to a hub. The torque limiter forms part of the gear reducer.